Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

247
FXUS63 KLOT 020130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
830 PM CDT Tue Apr 1 2025

.KEY MESSAGES...

- Waves of showers and storms will parade over the area tonight
  through much of Wednesday.

- Some storms may be severe tonight through Wednesday. The
  primary threats will be damaging hail and winds tonight
  through Wednesday morning, and then all hazards Wednesday
  afternoon (contingent on morning storms clearing).

- Gusty southeast winds (gusts up to 40 mph) will prevail
  overnight. Then, strong south winds (gusts in excess of 45
  mph) are possible Wednesday afternoon south of I-80.

&&

.UPDATE...
Issued at 830 PM CDT Tue Apr 1 2025

Overall, the forecast appears to be in good shape. Showers
continue to increase in coverage upstream of our area in
response to increasing isentropic ascent, well in advance of a
deepening cyclone in the central Plains. Coverage of thunder may
be relatively limited through daybreak as that is when the
MUCAPE axis (forced by a pocket of steep mid-level lapse rates)
will race northeastward into our area. The inherited forecast
handled these trends nicely.

There were two primary areas of focus for the evening update:

First, opted to modestly increase sustained winds and wind
gusts through the overnight period. While strong low-level
static stability beneath pronounced WAA will no doubt prevent
mixing into the base of what will be an intense low-level jet
overnight, the tightening low-level pressure gradient by virtue
of the approaching 988mb surface low and 1030mb high over
southern Quebec should allow for a commensurate increase in
wind magnitudes. For now, capped gusts in the 35 to 40 mph
range, highest north of I-88, in line with observed winds near
and west of the Mississippi River at press time. (Note that
there remains a window for wind-advisory criteria winds tomorrow
afternoon mainly along and south of I-80, conditional on the
blob of what will be nearly continuous showers and thunderstorms
surging northward by late morning. Opted to leave that period
unchanged, as realistically there is no better signal now than
before on coverage of convection by early afternoon).

Secondly, the evolution of the upper-level jet streak tonight
into tomorrow afternoon (rounding the "base" of the negatively-
tilted shortwave-length trough), presence of a strong
directional low-level shear to support the development of a
critical layer, and expansive west-to-east orientation of the
stable layer draped across the Lower Great Lakes north of the
warm front, will be favorable for the generation and
propagation of eastward-moving mesoscale gravity waves. With
such strong isentropic ascent atop the same surface stable
layer, do have to wonder if gravity waves may become aligned
with convection (gravity wave associated convection, or GWAC for
short) at times late tonight and especially after daybreak
tomorrow. Sometimes, GWAC can produce unexpected damaging winds
in spite of strong low-level static stability and opposing
synoptic wind directions. At this point, a majority of CAM
guidance doesn`t appear to show a signal for GWAC. However, the
18Z HRDPS showed a classic signal with equally-spaced
pressure/wind fields aligned with 1-hour QPF streaks.
Forecasting GWAC, let alone damaging-wind producing GWAC,
remains an immense operational challenge. So, confidence in it
actually occurring is low (call it a 5% or lower chance?). That
is all to say, the elevated convection late tonight through
early tomorrow afternoon may carry a threat for both damaging
hail and perhaps locally damaging winds, as well as localized
flooding due to training cells.

In terms of the severe weather threat tomorrow afternoon,
really don`t have much to add beyond what was discussed in Short
Term Discussion below. All will depend on how things look by
about 2 PM tomorrow afternoon in terms of any clearing or local
minima on ongoing convection.

Updated products will be sent soon.

Borchardt

&&

.DISCUSSION...
Issued at 308 PM CDT Tue Apr 1 2025

Through Wednesday Night:

Strong cyclogenesis is underway over the central high Plains
with a 988mb low near the Kansas/Colorado border early this
afternoon. A strong (130kt 250mb) Pacific jet is rounding the
base of the longwave trough over the western half of the country
and will be spinning up a vigorous mid-latitude cyclone from
the central Plains into the upper Mississippi Valley by
Wednesday afternoon.

Tonight into early Wednesday morning:

Strengthening warm front will begin lifting north across the
Mississippi Valley tonight as very strong 60-70kt southerly low
level jet develops in response to strong height falls. This
powerhouse low level jet will rapidly transport moisture
northward and upward steeply sloped isentropic surfaces
(290-300K). This extremely strong isentropic ascent should
result in increasing coverage of showers and thunderstorms
across northern IL and possibly NW Indiana late this evening
into the overnight hours. The highest chances of rain look to be
north of I-80 with lesser chances with southward extent. Hard
to pinpoint a southern terminus of the rain chances, so while
pops are lower across the southern CWA, there remains a chance
of showers/storms.

The rapid northward transport of this higher theta-e air mass
beneath moderately steep lapse rates should result in MUCAPE
values of 500 to locally over 1000 J/kg late tonight into early
Wednesday morning. Forecast soundings suggest effective shear
in the 30-40kt range coinciding with this moderate instability,
so it`s possible there could be some occasionally more intense
cells/transient supercells posing a threat of isolated
instances of hail 1"+ in diameter. While storm motion should be
very fast, there is a potential for some training of convection
which could result in some localized 1-2" totals in just an
hour or so. Most of our CWA could handle that type of rainfall
with just minor/nuisance type flooding, but should that
magnitude of rain occur in a more urbanized/poor drainage area,
then there would be a threat of some low end and localized flash
flooding.

Late Wednesday morning into early Wednesday evening:

Morning showers and thunderstorms are likely to slow the
northward advance of the warm front during the morning and
perhaps early afternoon hours, likely aided by the chilly
easterly flow off the lake. In fact, temps near the lake in the
immediate Chicago metro area could be in the upper 30s or lower
40s Wednesday morning into the early afternoon. Eventually, the
powerful synoptic low should overwhelm any lingering mesoscale
effects and allow the warm front to rapidly lift northward
across our CWA Wednesday afternoon. Temperatures will quickly
climb well into the 60s to lower 70s in the wake of the warm
frontal passage with dewpoints into the lower 60s, so a very
dramatic air mass change will take place Wednesday afternoon,
particularly near the lake.

Typically, nocturnal elevated convection lasting well into the
morning would prove to be pretty detrimental to afternoon
convective potential. However, in this case with such a powerful
cyclone to our north, there is concern that there could be
rapid recovery of the air mass during the afternoon. Most
guidance depicts and axis of 750-1500 J/kg of MLCAPE developing
ahead of the front that will be advancing across our area during
the late afternoon/early evening.

The forecast shear profiles are very concerning Wed afternoon
and evening with 40-50kt of south-southwesterly flow at 925 mb
veering to southwesterly at over 100kt at 500mb. This results in
very long and exceptionally favorable hodographs for
supercells. Very large, cyclonically curved hodographs in the
0-1km layer with mostly streamwise vorticity also would favor
the potential for tornadoes, especially given the low LCLs.

While the shear profiles are extreme, there remains considerable
uncertainty. First, does the forecast magnitude of instability
actually materialize as modeled tomorrow afternoon. Second,
convergence looks rather weak along the front and it is unclear
if it will be enough for convective initiation, assuming
sufficient instability. Finally, guidance generally has our CWA
in the convergent, right exit region of the strong upper level
jet.

Ultimately, there are many potential scenarios where we end up
with only weak, non-severe convection in the late
afternoon/early evening or no convection at all. Unfortunately,
given sufficient instability and intense convection develops,
the potential would exist for fast moving tornadic supercells,
with the potential for strong tornadoes.

Outside of the thunderstorm threat Wednesday afternoon,
southerly winds could gust to, or even over 45 mph Wednesday
afternoon south of the warm front. There are still uncertainties
as to how far north into our CWA that the potential for 45+ mph
gusts will extend, but seems plausible (likely?) that the
overnight shift will need to consider a wind advisory for at
least our southern CWA.

- Izzi

Thursday through Tuesday:

The surface cold front that shifts across our area on Wednesday
will stall out across the Ohio Valley for the second half of the
week. This front will become the primarily focus for several
additional rounds of heavy rain producing showers and
thunderstorms into the weekend as deep Gulf moisture continually
feeds into this frontal zone. A majority of this heavy rain is
expected to fall south of our area across southern parts of IL
and IN, especially on Thursday into Friday. Both days are thus
expected to remain largely dry across our area, with the
exception of some chances for light rain mainly south of I-80.

Late Friday through Saturday, several low amplitude impulses will
eject northeastward out of an upper-level trough/low shifting
across the Desert Southwest. Each impulse will aid in the
northward push of moisture over the surface frontal boundary, thus
supporting increasing rain (and perhaps a few embedded
thunderstorms) chances across much of our area Friday afternoon
into Saturday. Rain chances will peak in the 70 to 90% range
areawide Friday night, though the heaviest rainfall is currently
favored to fall south of I-80. A period of drier weather is
favored for the end of the weekend.

Temperatures through the period will remain seasonal, with
daytime highs generally in the 50s through Sunday. However,
onshore northerly winds Friday into the weekend will result in
cooler conditions (in the 40s) near the Lake Michigan shore. A
period of cooler (below average) weather also looks to be on tap
for the area early next week as a northern stream trough digs in
over the Great Lakes. This could support at least a day or two
with highs in the 40s and subfreezing overnight lows.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 700 PM CDT Tue Apr 1 2025

The key aviation weather messages are:

- Waves of showers and thunderstorms expected late tonight
  through early Wednesday afternoon. Some instances of dime to
  perhaps quarter-sized hail at times.

- IFR cigs developing late tonight into Wednesday morning. Brief
  period of LIFR conditions possible (30% chance) late Wed.
  morning/midday.

- LLWS threat as a warm front lifts north across the region.

- Additional round of storms possible late Wednesday
  afternoon/early evening (30-40 percent chance). Some of these
  could be severe.

- Strengthening east-southeasterly winds tonight. A few gusts
  towards 35 kts possible. Some uncertainty regarding timing a
  SW wind shift, but most likely in the 18-22z timeframe
  (earliest at GYY, MDW).

Precipitation/TS:

Showers will gradually expand across the airspace this evening
and overnight. After about 08z, the chances for embedded
thunderstorms will increase. Thereafter, several waves of
widespread showers and storms are expected to impact all the
terminals through Wednesday morning/midday. Latest trends in
guidance suggest that the main TS and precip chances may lift
north of the region during the early-mid afternoon hours, and
have commensurately reduced the TS mention during this period
with the latest TAFs. Thereafter, another period of TSRA is
possible during the 20-23z period at RFD, and 22-01z at the
Chicago-area sites (30-40 percent chances at this time). If
these storms do develop, they would pose a threat for all severe
hazards.

CIGs/Vsbys:

VFR will continue for most if not all of the evening hours. Cigs
are expected to build down to MVFR quickly after 06z, with
MVFR/IFR vsbys in SHRA/TS. There is a 30 percent chance for LIFR
cigs/vsbys late Wednesday morning/early afternoon. Any LIFR
threat still looks somewhat brief, and given the low confidence,
no mention has been included in the latest TAFs. VFR will return
Wednesday evening behind a cold front.

Winds and LLWS:

E to ESE winds will gradually increase this evening, with gusts
into the 25-30 kt range expected to become routine. A few gusts
of 35 to 40 kt can`t be ruled out late tonight/early Wednesday
morning, although confidence in this occurring is much too low
to justify a mention in the TAFs. Winds should then ease a bit
late Wednesday morning into the mid afternoon as a warm front
lifts northward. During this period, a LLWS threat (not
associated with TS) will materialize as a 55-60 kt SW low-level
jet develops with E/SE winds continuing at the surface. This
threat is mainly for the Chicago-area terminals where the LLJ
will be the strongest. There is some uncertainty in the timing
of a SW wind shift, but this generally looks to occur during the
18z-22z period; earliest at GYY and MDW and latest at DPA, ORD,
and RFD. Occasional gusts to 35-40 kt will be possible late
Wednesday afternoon.

Carlaw

&&

.MARINE...
Issued at 308 PM CDT Tue Apr 1 2025

- Gale Warning remains in effect from 1 AM to 6 PM CDT Wednesday

A deep surface low tracking toward the Upper Mississippi Valley
late tonight into Wednesday morning will tighten the pressure
gradient over southern Lake Michigan. The result will be
strengthening east-southeast winds. A period of east-southeasterly
gales remains possible overnight into Wednesday morning.

Then, as the deep surface low tracks toward northern Lake
Michigan later Wednesday, its warm front will lift north and bring
a shift to strong south-southwest winds Wednesday afternoon.
While cloud cover and ongoing showers and thunderstorms may limit
mixing depths on land, a period of gales into the first couple
miles of the nearshore will also be possible Wednesday
afternoon.

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 1 AM CDT Wednesday for the IL and
     IN nearshore waters.

     Gale Warning from 1 AM to 6 PM CDT Wednesday for the IL and IN
     nearshore waters.

&&

$$

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